Objectives The objective of this study is to examine the long-term effects of early social experience on behavior, cognitive function, HPA axis regulation, and psychobiological responses to stress in differentially-reared monkeys. ABSTRACT:As a joint cooperative venture between the MacArthur Foundation and the Asher Center, we have been conducting a number of follow-up studies on an existing cohort of rhesus monkeys that were exposed to variations in early experience as part of previous studies funded by the MacArthur Foundation Research Network on Depression. This cohort is comprised of monkeys that were generated from conditions in which their early experience was varied as part of a program of research entitled "Primate Models of Depression" ("PMOD"). In the original study, forty-eight rhesus monkeys were reared under two different social conditions from infancy, and later were either exposed or not exposed to stress in conjunction with pharmacologic treatments during the late infancy period (balanced across rearing groups). Animals in the altered early rearing group (peer-reared) were found to differ from controls (mother-reared) in their behavioral, hormonal and neurobiological profiles during the infancy and juvenile period. Follow-up studies have documented that behavioral, psychobiological and cognitive differences between the respective groups of monkeys persist into young adulthood. Keywords early experience, neurobiological development, HPA axis, biogenic amines, cognition, abnormal behavior